Solutions and methods for completely or partly removing lead, tin, or lead/tin coatings from the surface of another metal such as, for example, copper or nickel, such coatings having been deposited on such surfaces in an earlier process step, are needed, for example in the manufacture of printed wiring boards.
According to a known process, such an electro-chemically deposited coating, the latter conforming to the desired conductor pattern, serves as the etch resist.
According to another known process, a tin/lead coating is applied by the hot air levelling process. As a rule, the etch resist layer is subsequently removed again prior to the application of a solder resist mask.
For removing the etch resist, it has been proposed earlier to use solutions based on peroxide and containing inorganic acids such as hydrofluoric acid, fluoboric acid, or nitric acid. Furthermore, the use of peroxide-free solutions has been proposed, i.e., solutions containing, for example, thiourea, hydroxyphenols, alkali salts of m-nitrobenzosulfonic acid, or mixtures of fluoboric acid, hydrofluoric acid and nitric acid.
The solutions according to the state of the art have the serious drawback that they act on copper or nickel, which means they also remove material of the conductor track and costly process controls are required for keeping such removal, which is basically undesirable, as low as possible. In addition, the life of the known solutions is very limited, and they are unsatisfactory, economically speaking. Additional drawbacks include a highly exothermic reaction, as well as corrosion of the insulating and basic materials.